Friday afternoon, Nguyen walked away from the Royale Health Care Center, where he has been detained since December 2012.

Police said the 23-year-old told them he visited a friend while he was away from the facility, and it does not appear Nguyen committed any crimes while he was gone.

Authorities became familiar with Nguyen -- who has been diagnosed with schizophrenia and repeatedly detained and institutionalized -- in August 2011. At that time, he walked into the Westminster Police Department dressed in camouflage and holding an assault rifle, saying he wanted to "die by cop," Garden Grove Police Chief Kevin Raney said.

The weapon was not loaded, the chief said, adding that Nguyen did not have a permit for the firearm.

Nguyen was questioned, and police found he was fixated on a teacher at an elementary school he had attended in Northern California, Raney said, declining to name the school.

"We have statements and we have the firm belief that he is committed, unfortunately, to harming children at this specific school," Raney said.

Nguyen had been detained periodically since then, Raney said, until it became apparent in interviews with him in December that he was obsessed by the massacre that left 26 dead at a Newtown, Conn., elementary school on Dec. 14, 2012.

Nguyen has spoken repeatedly both about protecting children and endangering them, Raney said.

Nearly two weeks ago, Nguyen's mother, who lives in Huntington Beach, tried to gain custody of her son at a court hearing, but the judge refused to have him released, Raney said.